What caused major social changes in Pohnpei and Kosrae about seven centuries ago?

About seven centuries ago, major social changes in Pohnpei and Kosrae were driven by the development of centralized political and religious centers, specifically the construction of monumental "royal cities."

On Pohnpei, the construction of Nan Madol—an artificial city of islets—began around the 8th century, with significant monuments dating to 1180–1200. This site served as the seat of the Saudeleur dynasty, which unified rule over the island starting around 1100. Similarly, on the island of Lelu in Kosrae, a comparable royal city and feudal capital was constructed between 1200 and 1400.12

These developments marked a shift toward more pronounced social stratification in these regions. The elite occupants of these centers, such as the king and high chiefs, relied on the labor and tribute of their subjects to maintain their residences and religious sites. This transition established a hierarchical structure where power was concentrated in these urbanized, walled compounds, distinguishing the lives of the ruling class from the average population, who continued to live in dispersed homesteads.312